Properties of Heavy-Fermion Materials Demystified
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cate (TEOS), water, and ethanol mixed with molar ratios of 9/25/408/100. Subsequent hydrothermal synthesis at 100°C for 18 h resulted in a gel, which was aged for 24 h and later calcined at 480°C for 8 h. Particles thus obtained were 100 nm in diameter, and x-ray diffraction (XRD) revealed that the only crystalline phase present was silicalite-1. The round stainless steel sheet used as a substrate was 75 µm thick and 15 mm in diameter and had laser-drilled holes of about 80 µm in diameter. These holes, drilled by a Nd:YAG laser in pulse mode, were in a square lattice pattern a little over 200 µm apart. The pulse-drilling technique left rough edges around each hole on one side of the sheet. The silicalite-1 nanocrystals already synthesized were rubbed against this side of the substrate and thus filled the holes. The substrate was then placed vertically in a solution of KOH/ tetrapropylammonium bromide (TPABr)/ TEOS/H2O mixed with molar ratios of 1/1/4.5/100 in an autoclave. Hydrothermal synthesis at 170°C for 24 h resulted in a 30-µm-thick membrane of silicalite-1 crystals on the rough side of the substrate along with the silicalite-1 that
completely filled the holes in the stainless steel sheet. XRD showed silicalite-1 as the only phase present in the membranes. An array of micromembranes was capable of separating propane from N2, and had a permeance higher than the MFI-type current membranes. This is significant progress, the researchers said, since the membranes are essentially self-supported, with the zeolite in contact with both the feed and the permeate sides through a relatively small thickness. Changing the temperature or time of the hydrothermal synthesis varies the thickness of the resulting uniform silicalite-1 layer. When the researchers tried synthesis at 100°C, the thickness of the uniform silicalite-1 layer was less than 2 µm, while the perforations were still completely filled with silicalite-1. However, the thinner layer resulted in less desirable permeation properties. SIARI SOSA
Properties of Heavy-Fermion Materials Demystified Theoretical physicists Q. Si of Rice University and P. Coleman of Rutgers University, along with a team of experi-
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MRS BULLETIN • VOLUME 30 • FEBRUARY 2005
mental physicists led by F. Steglich at the Max Planck Institute for Chemical Physics of Solids in Dresden, have shown that the Fermi volume in materials with strongly correlated electrons changes its size abruptly at a “quantum critical point” as the temperature of the material is lowered near absolute zero Kelvin. “Quantum critical points are of great current interest because of their ability to reach up from absolute zero and create a new state of matter called ‘quantum critical matter,’” said Coleman. “This may provide a route to many new classes of material.” The research project, reported in the December 16, 2004, issue of Nature (p. 881; doi:10.1038/nature03129), addressed whether the Fermi surface transformation at the quantum critical point developed grad
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